1. Field of the Invention
The present invention relates to a method and an apparatus for polishing painted surfaces of a plate on which an overcoat of paint is applied.
2. Description of Related Art
Polishing of painted surfaces of car bodies is performed by using industrial robots, such as a teaching-play back type of industrial robots. These polishing robots have hands holding a polishing tool which are controlled based on information relating to position and posture of the hand so as to apply a specified polishing pressure to a painted surface. Such a polishing robot is known from Japanese Unexamined Patent Publication No. 60-20849.
In order to apply a specified polishing pressure to a painted surface in spite of position and posture of the robot hand, it is essential to use a great amount of information and a number of calculations. This rendered the polishing robot difficult to be manufactured at a low cost and small in size.
When polishing a paint coated surface of, for example, a car body, which is a free surface comprising flat surfaces and curved surfaces, it is essential to place a polishing tool held by a robot arm perpendicularly to the paint coated surface. However, there are sometimes cases in which the polishing tool must be vertically inclined but is placed perpendicularly to the paint coated surface according to angles at which the surface is inclined with respect to a horizontal plane. In such a case, only a component of the whole weight of the polishing tool is applied to the inclined surface. Accordingly, an effective load of the hydraulic cylinder, according to which polishing pressure applied to the surface by a polishing tool, changes. In other words, it is hard to maintain the polishing pressure constant.
Further, because portions of the surface to be polished are different in vertical level from one another, it is essential to make sure that a predetermined polishing pressure has been developed by downward movement of the polishing tool in order to provide a high quality of polishing over the surface. For this purpose, a moved distance of the polishing tool must be monitored. Accordingly, the polishing apparatus must be provided with a distance monitor and a pressure sensor which makes a control system complicated and takes a long time for moving the polishing tool due to distance measurement. This leads to difficulties in increasing polishing efficiency.
In order to provide a constant polishing pressure over a surface to be polished in spite of differences in vertical level, a moving mechanism of the polishing tool is controlled so as to develop a predetermined pressure applied from the polishing tool to portions of the surface even in different vertical levels. Such a polishing apparatus is known from, for instance, Japanese Unexamined Patent Publication No. 60-20849.
However, controlling the moving mechanism of the polishing tool against the surface makes the polishing apparatus complicated.